In the natural gas gathering industry, there have been two basic gas compressor units available in the 1000 to 6000 hp range. One is an electric driven gas compressor and the other is a reciprocating engine driven gas compressor. Electric driven gas compressors in this class range have been predominantly utilized in the offshore drilling industry and, in isolated cases, for land side use.
An electric driven gas compressor offers more favorable maintenance features. However certain issues regarding the cost of electricity and related issues regarding necessary amount of reserve capacity or standby available power required to start an electric motor have prevented electric driven gas compressors from being more widely utilized.
Most electric utilities in the US consider the availability of extra capacity when establishing an electricity rate to large industrial users of electricity. Large industrial users are often assessed penalties in the form of higher electricity prices for peak electricity demands exceeding base load requirements, especially when the industrial user requires instantaneous and short spikes, as for example, when starting an electric motor. Starting an electric motor may require up to a 650 percent increase in the normal operating demand for current by the electric motor. Since this current demand requires the utility to have the extra capacity in reserve, this reserve or peak load demand is more expensive to provide. In most cases, the extra cost of the electric current makes use of electric motor driven gas compressors less feasible than running the gas compressor on a reciprocating gas engine.
Consequently, reciprocating gas engine-driven compressors are used in the majority of gas compressor stations. A portion of the natural gas forwarded at the gas compression station is utilized to operate the natural gas fired engines which drive the gas compressors. Due the higher cost of a lower rpm reciprocating gas fired engine, most gas compressor units utilize high rpm reciprocating gas engines in the compressor packages. Operations and maintenance cost of the reciprocating gas engine is usually high and constitutes a large portion of the cost of operating a gas compressor. Unscheduled down time due to unexpected engine failures are a common complaint. Major overhauls of the engine are costly and are frequently required.
An electric motor driven compressor, requires much less maintenance and provides increased run time over reciprocating gas engine driven compressors, and would be even more feasible if the price of electricity were lower.